SUNSTROKENinth-ranked Serena Williams blamed her left knee for her 6-2, 7-6 (7) defeat at the hands of wild card Sun Tiantian, ranked 127th in the world

Sun Tiantian of China reacts to an umpire's decision during her second-round win over Serena Williams of the US at the China Open tennis tournament on Wednesday.

PHOTO: AFP

Defending champion Serena Williams lost her opening match at the China Open on Wednesday, falling to 127th-ranked Chinese wild card Sun Tiantian by 6-2, 7-6 (7).

It's the first time 9th-ranked Williams, winner of seven Grand Slams, has lost to anyone outside the top 100 in a main draw. It was Sun's first win over anyone in the top 40.

Williams hasn't reached a final since winning the Australian Open in February, and has been bothered by shoulder, ankle and knee problems.

"I would say the main reason for my loss today is my left knee," said Williams, who had a first-round bye.

"I just played terrible. Sun played very well and was ready for the match. I'm disappointed I couldn't perform better today," she said.

Sun startled the home crowd by jumping ahead 2-0 and easily taking the first set. In the error-filled match, Williams frequently sprayed balls long, hit drop shots short, and double-faulted. She was broken to trail 4-5 in the second set, broke Sun back in the next game, but Sun prevailed in the tiebreaker 9-7.

"Before the match I was just saying don't be nervous," said Sun, through to only her second career quarterfinals.

"I had an opportunity today and I grabbed it. I just hope I can keep improving. My goal for this year is to crack the top 100," she said.

Williams, whose injuries forced her to miss the French Open and didn't help in her earliest defeats at Wimbledon and the US Open since 1998, said she was keen to rest and recover.

"I definitely think I'm going to get well before I perform again, for sure," she said.